1. Field of the Invention
The present invention is related to a reading/writing apparatus used in a non-contact IC card system. More specifically, the present invention is directed to a non-contact IC card reading/writing apparatus capable of improving a power transfer efficiency to a non-contact IC card, and of improving a data reception efficiency from a non-contact IC card.
2. Description of the Related Art
In general, reading/writing systems with employment of IC cards are conventionally referred to as non-contact IC card systems. These conventional non-contact IC card systems have been gradually and practically utilized in physical distribution systems, traffic systems, air cargo management systems, and the like, which use a frequency band of, for example, 13.56 MHz.
Now, FIG. 4 is an explanatory diagram for explaining a conceptional idea of a conventional non-contact IC card system. As indicated in FIG. 4, this conventional non-contact IC card system is arranged by a non-contact IC card 101 (will be simply referred to as “IC card” hereinafter), and a reading/writing apparatus 105 which is communicated with this IC card. The non-contact IC card 101 is equipped with both an IC chip 103 and an antenna coil 102 on one sheet of such a card made of resin. This reading/writing apparatus 105 is equipped with a loop antenna 104. Both electric power and transmission data are continuously, or intermittently transmitted by this loop antenna 104, and reception data transmitted from an IC card is acquired by the loop antenna 104, which is located within such a range that this electric power and the transmission data can be received by this IC card.
As one example, the reading/writing apparatus of the non-contact IC card system described in (Japanese Laid-open Patent Application No. 2002-007976) is shown in FIG. 5. FIG. 5 is a block diagram of the conventional IC card reading/writing apparatus. FIG. 5 indicates a portion related to coupling between a reading/writing apparatus 111 and a non-contact IC card 112 of the above-described conventional non-contact IC card system.
Firstly, in the case that transmission data is transferred, a carrier wave produced from an oscillator 106 is entered to a modulator 107, and the modulator 107 modulates this entered carrier wave by data “DATAa.” Then, the modulated carrier wave is amplified by a power amplifier 108, and the amplified carrier wave is transmitted via a matching circuit 109 from a loop antenna 110.
Also, in the case that only electric power is transferred, the carrier wave produced from the oscillator 106 is transmitted in a non-modulated carrier mode. The transmission of the non-modulated carrier wave from this reading/writing apparatus 111 to the non-contact IC card 112 is carried out by that magnetic fluxes produced from the loop antenna 110 magnetically intersect the antenna coil 102 of the non-contact IC card 112 so as to energize an induced voltage due to an electromagnetic coupling effect. On the side of the non-contact IC card 112, the induced voltage of the antenna coil 102 is rectified by a rectifying circuit (not shown) employed in the IC chip 103, and thus, the rectified voltage is employed as a power supply with respect to the respective circuits employed in the non-contact IC card 112. Also, the same induced voltage is conducted to a demodulating circuit (not shown) so as to demodulate data supplied from the reading/writing apparatus 111.
Next, when data is transferred from the non-contact IC card 112 to the reading/writing apparatus 111, the reading/writing apparatus 111 transmits non-modulated carrier waves so as to supply only electric power to the non-contact IC card 112. On the side of this non-contact IC card 112, in response to a “1” bit and a “0” bit of data “DATAb” read out from a memory (not shown) provided in the IC chip 103, a switch is turned ON/OFF in a modulating circuit (not shown) which is constituted by this switch and a load resistor (not shown), which are connected to, for example, the antenna coil 102. As explained above, when the switch is turned ON/OFF, a load with respect to the antenna coil 102 is varied. This load variation is transferred to the loop antenna 110 provided on the side of the reading/writing apparatus 111 due to electromagnetic induction effects, and thus, an impedance on the side of the loop antenna 110 is varied, so that a voltage/current value, namely an impedance at a point “A” of the reading/writing apparatus 111 is changed in response to the transmission data “DATAb” of the non-contact IC card 112. As a result, an amplitude of a high frequency signal is varied. In other words, this high frequency signal is amplitude-modulated by the data of the non-contact IC card 112. This modulated high frequency signal is demodulated by the demodulating circuit 114, so that the data “DATAb” is obtained.
First prior art is shown in FIG. 5(b). FIG. 5(b) is a diagram for indicating a detailed input portion of the demodulating circuit 111 shown in FIG. 5(a). As previously explained, when the data is transmitted from the non-contact IC card 112, a load “z” of the antenna coil 102 of the non-contact IC card 112 is changed based upon the data DATAb. As a result, an output current “I” of the power amplifier 108 is changed. Thus, in order to detect this current change, a resistor 115 is inserted into the ground side of the loop antenna 110, and a voltage drop which is produced by that the output current “I” flows through this resistor 115 is entered to the demodulating circuit 114. The demodulating circuit 114 detects a change contained in the inputted voltages so as to demodulate the data “DATAb” from the non-contact IC card 112. However, when the current “I” outputted from the power amplifier 108 flows through the resistor 115, the electric power is consumed in this resistor 115. As a result, the power amplifier 108 requires extra output power which is equivalent to such an electric power consumed by this resistor 115, so that a power transfer efficiency is lowered.
In this case, FIG. 6 is a block diagram of a conventional non-contact IC card reading/writing apparatus. As second prior art, FIG. 6(a) indicates a detailed peripheral portion of a demodulating circuit when parallel resonance is employed, in which a capacitor 116 and the loop antenna 110 are operated under parallel resonant condition.
In this case, since an impedance of a parallel-resonant circuit becomes a large impedance value in the vicinity of a resonant point, an output-sided impedance of a matching circuit 109 becomes a large impedance value in conjunction with the above-described large impedance value. Then, a voltage “V” of this high impedance point is captured via the resistor 117 to the demodulating circuit 114 so as to be demodulated. In this circuit arrangement, a series impedance which is constituted by the resistor 117 and the input impedance of the demodulating circuit 114 in the carrier wave band is connected parallel to such a parallel circuit which is constituted by the loop antenna 110 and the capacitor 116. Therefore, in order to detect the data transmitted from the non-contact IC card 112, a Q-factor of the resonant circuit is lowered. This phenomenon may immediately lower a power transfer efficiency with respect to the non-contact IC card 112.
Also, FIG. 6(b) is a circuit diagram of a peripheral circuit portion of the demodulating circuit 114 as third prior art in the case that a power transfer efficiency is improved by way of series resonance. In this third prior art, both the loop antenna 110 and the capacitor 118 constitute a series-resonant circuit. When series resonance occurs, since an impedance of this series-resonant circuit represents a small impedance value, a current “I” derived from the power amplifier 108 is supplied to the resistor 119, and thus, a voltage drop occurred in this resistor 119 is detected by the demodulating circuit 114. As a consequence, electric power is consumed in the resistor 119. Furthermore, in this case, since the resistor 119 is series-connected with respect to the series-resonant circuit, the Q-factor of the series-resonant circuit is lowered, so that the power transfer efficiency is lowered.
In addition, FIG. 6(c) indicates a circuit arrangement as to an input unit of the demodulating circuit 114 and a peripheral circuit thereof, as fourth prior art. In this fourth prior art, the power amplifier 108, a parallel-resonant circuit 121, and the demodulating circuit 114 are coupled to each other by employing a matching transformer 120 having windings “n1”, “n2”, and “n3.” In this circuit, a turn ratio of the winding “n1” to the winding “n2” is set to such a value which is matched by a resonant frequency between the output of the power amplifier 108 and the parallel-resonant circuit 114. A coupling operation between the demodulating circuit 114 and the parallel-resonant circuit 121 is carried out by this matching transformer 120. Then, a turn ratio of the winding “n3” to the winding “n2” is arranged in such a manner that a matching condition may be established in the frequency band of the data “DATAb” transmitted from the non-contact IC card 112. However, in this circuit arrangement, an insertion loss of the matching transformer 120 occurs, so that the power transfer efficiency is lowered.
Moreover, in any circuits of the first prior art to the fourth prior art, there is no directivity as to transfer directions of the electric power, the transmission data, and the reception data. That is, in any circuit arrangements of these prior art, the output power derived from the power amplifier 108 may be supplied to any of the loop antenna 110 and the demodulating circuit 114. As a result, the electric power radiated from the loop antenna 110 to the spatial area is lost by such an electric power which is supplied to the demodulating circuit 110, so that the power transfer efficiency is lowered.
Also, in the case that the reception data is acquired, the change contained in the load impedance values at the loop antenna 110 may be transferred to both the demodulating circuit 114 and the power amplifier 108. As a result, the transferred changing component of the load impedance from the loop antenna 110 is lowered on the side of the demodulating circuit 114.
Furthermore, since the high frequency signal having the large amplitude derived from the power amplifier 108 is supplied to the demodulating circuit 114, there is another problem. That is, such a filter circuit having a high-performance band blocking characteristic must be provided at a prestage of the demodulating circuit 114 in order to filter this high frequency signal having the large amplitude.